The atria of the heart synthesize a family of diuretic/natriuretic/vasodilator peptides called atriopeptins. The purpose of this investigation is to study hemodynamic changes in response to intravenous exogenous synthetic atriopeptin III. The goal is to learn more about the physiology of atriopeptin III under various conditions to determine when this hormone may be important in the regulation of blood pressure and cardiac output. Blood flow changes will be monitored by means of directional pulsed-Doppler flowmetric technique simultaneously in renal, mesenteric and hindquarter (muscle) resistance beds. Blood pressure and heart rate will also be monitored simultaneously and regional changes in resistance in the renal, mesenteric and hindquarter vascular beds will be calculated during each cardiac cycle for the duration of the response to atriopeptin III. In other rats, the pulsed-Doppler flowprobe will be placed on the ascending aorta to measure changes in total cardiac output and total peripheral resistance. Experiments are designed to compare the responses to atriopeptin III in states of water deprivation vs normal hydration vs volume load and under conditions of sodium deprivation vs normal sodium intake vs high sodium intake and in animals that are being infused chronically with aldosterone, with antidiuretic hormone, or with adrenocorticotropin. Plasma levels of atriopeptin III will also be determined under each of these physiological conditions before the administration of exogenous atriopeptin III. It is important to understand more about the cardiovascular physiology of the atriopeptin system in order to eventually study its possible role in the pathophysiology of certain sodium-sensitive or volume-sensitive forms of hypertension, congestive heart failure, renal insufficiency and endocrine disorders.